Generally, a vehicle-mounted car navigation system has a route guidance function. During the route guidance, let us suppose that a vehicle position approaches a specified guidance point such as an intersection for right or left turn or an entry to or an exit from an express highway. In such case, a screen display on a display device changes to an enlarged map near the intersection. In addition, the system provides voice guidance such as “turn to the left 300 m ahead” or “turn to the left ahead.”
Such voice guidance is effective because it enables the guidance without necessitating a driver to view the display screen. On the other hand, the driver (or passenger) may hardly hear the voice guidance due to sounds outside the vehicle (e.g., the other vehicle's horn sound). Reversely, the guidance voice may make it difficult to hear sounds outside the vehicle. A fellow passenger may feel the guidance voice annoying while he or she is sleeping or listening to the music.
In recent years, several techniques are proposed to provide the driver with more easily recognizable guidance at guidance points. The techniques aim at attaching a vibrating body to a driver's seat or a steering wheel to transmit vibration to the driver's body (back or hands) for providing the guidance (e.g., see patent documents 1, 2, and 3).                Patent document 1: JP-2000-221051A        Patent document 2: JP-2000-155893A        Patent document 3: JP-2004-170359A        
When the driver's seat or the steering wheel is vibrated as mentioned above, however, the seat or the steering wheel needs to contain the vibrating body and special wiring. The device becomes large-scale, requires complicated installation works, and greatly increases costs. Vibrating the seat or steering wheel makes it difficult for the driver to determine whether the guidance instructs the driver to turn to the left or right. Further, some drivers may be surprised or displeased at sudden vibration.